Method and apparatus for hindering interference with wireless transmission of messages



Oct. 13, 1942. F. c. P. HENROTEAU 2,298,562

METHOD AND APPARATUS FOR HINDERING INTERFERENCE WITH WIRELESS TRANSMISSION OF MESSAGES Filed May 13. 1941 Jwzwrae, fkA/vcals C. Ema-#2075: 0

Arraxavsr Patented Oct. 13, 1942 UNITED STATES PATENT OFFICE Francois Charles Pierre Henroteau, Ottawa, Ontario, Canada Application May 13, 1941, Serial No.

In Belgium May 11, 1940 19 Claims.

This invention relates to a method and apparatus for hindering interference with wireless transmission of messages, that is, for preventing what is commonly known as jamming of wireless messages. Throughout this specification the word transmission is used to indicate the whole operation of getting a message from a sender to a receiver, the word emission being reserved for that part of the transmission which is composed of the operations at a sending station.

Many systems have been proposed for the sending of secret signals by wireless. However, all transmissions, except those on very short Wave lengths of the order of ten centimetres, are subject to interference by jamming with signals sent out from another source on the same wave length. Thus in War no system for sending secret signals can be of much use when applied to ordinary wave lengths, because of the fact that such signals can be jammed .by the enemy and their reception rendered impossible. While this diffi culty does not arise with the very short wave lengths referred to, such wave lengths are subject to the practical limitation of being stopped by substantially those subjects which will stop light, and thus have a very short range particularly in close country.

According to the present invention a method is provided which permits the transmission of signals in a form in which they are unintelligible and cannot be jammed. According to the invention, the method comprises distorting a filterable wave to an unfilterable one, modulating a carrier wave with such distorted wave itself modulated in accordance with a message, particularly a Morse type message, and simultaneously modulating the carrier wave with a different unfilterable wave. The filterable wave will normally be one of constant frequency and will be distorted to one of variable frequency, preferably to a substantially aperiodic wave, and similarly the other unfilterable wave will be one of variable frequency and preferably substantially aperiodic. In order to ensure that the variations in amplitude of the wave carrying the message should not betray that message, the other unfilterable wave is preferably of varying amplitude, the variations, when Morse transmission is in question, being of the nature of those obtained by impressing a Morse type message thereon.

More specifically, the method in question comprises recording a wave of constant frequency, converting the resulting record at a variable speed into an electric wave of variable frequency,

modulating a carrier wave with this electric wave 55 means for scanning the an electric wave of Figure 3 is an key to 'distort a wave of 50 are unintelligible.

5 The method of the invention may be carried out in apparatus which comprises a mosaic, means for producing a beam of electrons, means for varying the intensity of this beam in accordance with an electric wave of constant frequency, mosaic with the beam to form an electrostatic record of the latters variations in intensity, means for scanning the mosaic at a speed variable with respect to the speed of recording to convert the record into variable frequency, means for causing the wave of variable frequency to be modulated in accordance with the message, means for producing another wave of differently variable frequency, and means for modulating 0 a carrier wave simultaneously with these waves of variable frequenc The following detailed description deals with the transmission of Morse type messages in acits most useful application. The description is given by reference to the attached drawing, and

from it additional features of novelty will appear.

In the drawing,

30 Figure 1 is a diagrammatic illustration of an emitting apparatus constructed in accordance with the invention,

Figure 2 is a face view of a key plate used in such apparatus,

enlarged detail of part of the key plate, and

Figure 4 is a diagrammatic illustration of a receiver.

The invention is based on the idea of using a a given constant frequency into one of variable frequency and modulating this wave With a message at the emitter before or after distortion, and then at the receiver using the same key to reverse the process of distortion and return the wave of variable frequency to one of constant frequency which can be filtered out from all other signals received. It will be clear that, so long as the form of the key is unknown to an enemy, the emitted signals Moreover, no signal can be sent out which will jam them, because any signal other than one whose frequency varies exactly in accordance with the key will be converted at the receiver into a wave which is not of constant frequency and will thus be rejected by a filter that the electron beams in rate and causes the i of electrons which are directed towards and Z designed to pass only a wave .of thegiven con; stant frequency. However, the period of any key which could bepractically used is necessarily comparatively short. Consequently if no further steps weretaken than those just mentioned an enemywho recordedemitted signals over a considerable period of time equal to a large number so that the plate It will assume a negative potential and the beam l a will be caused to move towards the upper end of the mosaic 8. The amount of key periods might eventually be able todetermine the pattern of the .key. It is in order to prevent any suchpossibility'that the other wave of differently variable frequency, which has been referred to above, is used.- As this wave is simply a covering wave and is rejected at the receiver by the filter for the wave of given constant frequency, it can be of any desired irregularity, and

is preferably substantially aperiodic, so that dlS- covery of the repeating key becomes thus impossible'for an enemy.

The form of emitter according to the invention 1 which is illustrated in Fig. 1, comprises an emitting tube 5 containing a plate 1 of material capable of emitting electrons and a mosaic 8, the term "mosaic wherever used in this speciflcation meaning a device of the general type used in television apparatus such as the iconoscope and composed of a multiplicity of mutually insulated tiny conductive electron emissive but not necessarily photosensitive elements (represented by the'dotted line 8a) mounted on and insulated from a signal plate (represented by the solid line 8b). Associated with the plate I is an electron gun 9 and associated with the mosaic 8 is an electron gun), each gun being contained in an appropriate pocket in the tube 5. For purposes of simplicity, 'each gun is shown as provided with only one pair of deflecting plates so plate 1 and mosaic 8 respectively along a single line only. I

For the purpose of modulating the intensity of the electron beam 9a a modulating electrode H is associated with the gun and is connected through a Morse key 6 to two oscillators I21; and I2!) which produce two waves (two being used instead of only one for a purpose hereinafter described) both of constant but preferably quite widely different frequencies. The intensityof the beam 9a thus varies in accordance with the resultant of two constant frequency waves modulated in accordance with a Morse message formed by operation of the Morse key 6. The beam 9a with its intensity thus varied scans the plate I at a reguemission from the latter focussed on the mosaic 8 by an accelerating electrode l3 and focussing coil M. The action of these electrons on the mosaic produces in the manner well understood an electrostatic record of variations of intensity of the beam 9a and therefore a record of the waves produced by the oscillators I20. and i212 as modulated by operation of the Morse key 6.

The electron beam Ilia, unlike the beam 9a, is of constant intensity but, on the other hand scans the mosaic 8 at an irregular rate of speed. In the drawing, oneof the deflecting plates 15 for it is shown as grounded and the-other plate I8 is connected through an amplifier to the anode ll of a photoelectric cell l8. When no current passes in this cell the arrangement is such that the beam i001 strikes the lower end of the mosaic 8.

It will be seen, upon the cathode I80. will receive electrons and become more negative,

however, that when light falls irregular rate, from the point where it is at a of light'falling on the cathode We is caused to varyin accordance with, a periodically repeated pattern by means of a cathode ray oscillograph l 9, and a key plate '20. The cathode ray oscillograph is provided with a fluorescent coating 2| over which a beam of electrons l9a'is caused to move in a circular path in any suitable way such as by connecting one deflecting plate of each pair of such plates to ground and the other plate of each pair to an appropriate source 22 of alternating potential.

A face view of key plate 20 is shown in Figure 2 from which it can be seen that the plate has on it a circular path corresponding in size to the circular path described by the luminous spot produced by movement of the electron beam I8a over the fluorescent coating 2|. The translucency pathincreases constantly, though at an i minimum to the point 28 just short of 360 around 9a and Illa scan the of the cell, the anode II the circle where it is at a maximum, and then decreases suddenly to the minimum again. In the key illustrated the line' 21- represents the boundary between the opaque portion of the path and the translucent portion, the irregularity of this line being shown in Fig. 3 in somewhat more detail than=is possible in Fig. 2.

A lens 28 is provided to focus on the cathode 18a any light which passes through the key plate 20. The result of the operation of the cathode ray oscillcgraph is that the amount of'light falling on the cathode I80. of the photoelectric cell I8 constantly increases at an irregular rate in the time which it takes for'the luminous spot on.

the fluorescent coating 2| to make a complete circuit of the path 24. The apparatus is so arranged that the maximum light falling on the cathode I1 is suflicient to bring the deflecting plate IE to such a potential as to direct the electron beam llJa at the upper edge of the mosaic 8. The sudden decrease in the amount of light ialling on the cathode l8a when the luminous spot passes from a point opposite the point 26 to one opposite the point 25 causes a suddenreduction in the potential of the plate l6 and the return of beam l0a to its initial position.

Because of the irregularity in the rate of increase of the translucency of the path 24 and the consequent irregularity inthe rate of increase of the potential of the deflecting plate It, the beam [0a. will not move at a constant speed across the mosaic 8 but at a speed varying in accordance with the pattern of the key 20. Movement of the beam Ilia across the mosaic results, in the usual way, in the erasing of the electrostatic record formed on the mosaic and the conversion of that record into an electric wave which passes through the wire 29. This electric wave, however, because of the irregular speed of conversion of the record is of a variable frequency governed by the periodically repeated pattern of the key 20. It will be appreciated, moreover, that as a result of the irregular speed of scanning by the'beam Illa the duration of the portions. of the electric wave passing through the wire 29 corresponding to Morse dots and dashes will be distorted. Thus, for example, assume that the record of -a Morse dash is three times as long as the record of a Morse dot. If, during the time that the record of a Morsedash was being erased from the mosaic 8 the beam Illa were moving three times as fast as when the record of a Morse dot was being aaoasoa erased the durations of the portions ofthe electric wave passing through the wire 29 corresponding to this Morse dot and Morse dash would be equal.

The electric wave of variable frequency is fed to an amplifier and modulator 3| where it is used to modulate a carrier wave which is emitted from the antenna 32.

The formation of the record on the mosaic 8 will produce an electric wave which will flow along the wire 29 and may for convenience be referred to as the recording wave to distinguish it from the wave of variable frequency mentioned above which can conveniently be described as the erasing wave since it results from the erasing by the beam Illa of the record on the mosaic 8. This recording wave being an exact reproduction of the compound wave applied to the modulating electrode it, its emission must be prevented because otherwise being of constant frequency it could be filtered out of the emitted signal by an enemy and the message read. Its emission could be prevented, for example, by using two identical tubes and associated devices, both connected to a single amplifier and modulator 3|, recording and erasing alternately, and providing means for blocking passage of current to the device 3| from the tube in which recording is being done as shown in United States Patent No. 2,191,565, particularly in Figure 2. However, the result can be conveniently attained without duplication of apparatus. For this purpose the speed at which the electron beam 9a scans the plate I should be either higher than the highest speed at which the beam we moves during scanning or lower than the lowest such speed. In the apparatus illustrated the latter has been taken to be the case. The result of the relatively fast scanning by the beam Illa will be that the lowest frequency of the erasing wave passing through the wire 29 will be higher than the highest frequency of the recording wave so that the two waves may be separated by a filter. It is for this purpose that the high pass filter 3D is shown in the line 29, its values being so calculated as to allow the erasing wave to pass but to block the recording wave. When a number of constant frequency waves, for example two as shown, are used, then the lowest erasing scanning speed multiplied by a fraction, of which the numerator is the lowest constant frequency and the denominator, the highest constant frequency impressed on the modulating electrode It must be higher than the recording speed, otherwise some of the frequencies in the erasing wave might not pass the filter 30. Of course, the erasing will take, in the circumstances, only a fraction of the time taken for recording and during the remainder of the time required for the beam 9a to perform its traverse of the plate '5 the beam Illa will be inactive. The period, however, between the beginning of one traverse by the beam Illa and the beginning of the next traverse by that beam will be the same as the period between the beginning of one traverse by the beam 9a and the beginning of the next traverse by that beam.

The device 3| besides being connected to the signal plate 8b through the wire 29 is connected to the output of two similar photocells 13, on the cathode M of each of which falls the light which passes from a source 15 through a film l and mask l1. Each film I6 is a record preferably of an aperiodic wave, and appropriate means are provided for running each record past the mask ll during the operation of the emitter. Conseiii quently the wire 18 connecting the output of the cells 13 to the device 3! carries a compound wave formed of the two aperiodic waves recorded on the films I6. The purpose of having two records is to avoid the necessity of having either of them very long, because by using two and occasionally changing the relation to each other of the starting points of the two records the practical effect of a single record of great length can be obtained. Thus, for example, if the running time of one record were three minutes and that of the other five, and both started together at the beginning, fifteen minutes would elapse before the resultant compound wave carried by the wire 18 repeated itself. If before the end of this time one record were stopped for a moment while the other was" allowed to proceed, repetition at the end of the first fifteen minute period would be prevented and a new period started. Thus by taking two records whose lengths have a comparatively high lowest common multiple and occasionally changing what may be termed by analogy their phase relationship, the compound wave carried by the wire 78 will for all practical purposes be aperiodic. This wave, like that from the signal plate 812 is amplified and modulated in the device 3! on the same carrier wave as that on which the wave carried by the wire 29 is modulated. The carrier wave thus modulated by the two waves of variable frequency is emitted from the antenna 32.

It may be desirable to hide the sharp variations of amplitude in the wave passing to the device 3! from the signal plate 8b and resulting from the making and breaking of the connection between the oscillators In and I2!) and the electrode I I through the Morse key 6, since by an analysis of these sharp variations the message might be discovered. Accordingly, the records on the films 16 should be made of varying amplitude and preferably with the same sharp variations of amplitude as would be obtained by the impression of Morse signals thereon. These variations would of course be distributed irregularly rather than in accordance with Morse signals having any meaning. It may be here pointed out that, if desired, the output of the photocells 13 may be connected to the modulating electrode it and thus be passed through the tube e and distorted by the variable speed scanning. The arrangement shown, however, is simplerand just as satisfactory.

The modulated carrier Wave emitted from the antenna 32 is picked up at the receiving antenna 33 and passed thence through a demodulator and amplifier 3t, whence the received wave of the variable frequency identical with the erasing wave at the emitter passes through a wire to a modulating electrode 36 for controlling the intensity of an electron beam 31a produced by an electron gun 37 in a receiving tube l0 similar to the tube 5 at the emitter. The beam 37a scans a plate 38, similar to the plate 1 in the tube 5, at a speed which varies according to a pattern identical with that according to which the speed of scanning of the beam Illa in the tube 5 is varied. and in a manner similar to that described in connection with the tube 5. The scanning of the plate 38 by the beam 37a results in the formation of an electrostatic record on a mosaic as similar to the mosaic 8. Thus, the received wave of variable frequency is recorded on the mosaic 39 but in a distorted form as a result of the variable scanning speed of the beam 310.. The scanning movements of this beam are governed by an arrangement similar to that which governs the scanning movements of the beam Ma. One of the deflecting plates 40 isgrounded and the other plate 4| is connected to the cathode 42 of a photoelectric cell 43 similar to the cell l8. Associated with the cell 43 is a cathode ray oscillograph 44 having a fluorescent screen 45 and in which an electron beam 44a moves in a-circular path synchronously with the beam l9a in the oscillograph i9, the movement of the beam 44a being controlled by an oscillator 22a synchronized with the oscillator 22. The light from the rotating luminous spot produced by the action of the beam 44a on the fluorescent coating 45 strikes a key plate 23 identical with the key plate 20 and passes therefore. through a path on the plate 23 identical with the path 24 described above. Light passing through this path is focussed by a lens 46 on the cathode 42.

If a given portion of the received electric wave reaches the modulating electrode 36 at a time when the beam 31a is performing a part of its traverse of the plate 38 corresponding to the part of the traverse of the mosaic 8 which was being performed by the beam Illa when the portion of the erasing wave corresponding to the given portion of the received wave was being produced, then, if the beam 31a, is scanning exactly in step with the beam Inc, the electrostatic record formed on the mosaic 39 will be identical with that formed on the mosaic 8. This will be apparent from a consideration of a simple example, the values mentioned-being taken simply for purposes of explanation and-having no relation to actuality. Suppose that the total time required for the beam Hia to traverse the mosaic 8 from the bottom to the top is one second and that one traverse of the plate I by the beam 9a results in the recording on the mosaic 8 of twelve complete cycles. If the beam lOa traversed the mosaic at a constant rate, the frequency of the erasing wave would be twelve cycles per second. Assume however, that the beam Illa covers the lower half of the mosaic in $4; of a second but takes of a second to traverse the upper half. It thus erases the record of six cycles in the first quarter of a second so that the frequency of the electric wave passing through the wire 29 for the first quarter of a second is twenty-four cycles per second. In each of the last three quarters of a second the beam erases the record of only two cycles so that the frequency of the electric wave passing through the wire 29 for the last three quarters of a second is only 8 cycles per second. Now assume that the'beam 37a is moving at exactly the same variable rate of speed as the beam a and that it accordingly traverses the lower half of the plate 38 in A, of a second and the upper half in of a second. If, just as it is about to begin to traverse the plate 38, the front of an electric wave corresponding to that produced during the traverse just described of the mosaic 8 by the beam Illa reaches the modulating electrode 36, the result will be that during the first quarter of a second the intensity of the beam 31a will go through six cycles of variation. Consequently there will be formed on the lower half of the mosaic39 a record of six cycles. During the next three quarters of a second the intensity of the beam 31a will likewise go through six cycles of variation, so that there will be formed on the upper half of the mosaic 36 a record also of six cycles. The complete record on the mosaic will thus be identical with the record which was erased from the mosaic 8.

. ing the described traverse of the mosaic 8 by the beam Illa were to reach the modulating electrode 36 second after the front of the wave to which it corresponds had left the signal plate 8b, and the beam 3111' were scanning exactly in step with the beam Ma, the former would already have traversed the lower half of the plate 38 and would be about tobegin the slow part of its traverse. In

the next quarter of a.'second, during which it would traverse only one-sixth of the total length of the plate 35, its intensity would go through six cycles of variation and consequently a record of six cycles would be made on one-sixth of the length of the mosaic 39. Duringeach of'the next two quarters of a secondthe intensity of the beam 31a would go through two cycles of variation, and consequently two cycles would be recorded on each of the upper two. sixths of the length of the mosaic 39. 'If the resulting record were scanned at a constant speed the resulting electric wave would be of variable frequency.

If, however, the key plate 23 were 2 turned through an angle of 90 with respect to'the key plate 20, then the beam 31a, would start its traverse of the plate at y second later than the beam illa started its traverse of the mosaic 8.. Consequently, 'the beginningof the traverse of the plate 38 by the beam 3M would, in the example just discussed, coincide with the arrival at the modulating electrode 35 of the front of the electric wave generated during the first-quarter of a second of the movement of the beam Illa, so that the record formed on the mosaic 39 would be of a wave of the same constant frequency as thatgenerated by the oscillator l2.

The electrostatic record formed on the mosaic 39 is scanned periodically by a beam 4911 from an electron gun 49, this beam being moved exactiy in step with the scanning of the plate 7 by the beam 9a. The scanningby the beam 49a erases the electrostatic record from the mosaic 39 and converts it into an electric wave which, in the simplest form of apparatus when only one wave of constant frequency was used at the emitter, passes from the signal plate 39a through a band pass filter designed to permitthe passage only of a constant frequency wave identical with that produced by the oscillator at the emitter, any current which passes this filter being fed to, for example, a pair of head phones.

As shown in the drawing, the key plate 23 is rotatably mounted on a central support 23a and is provided with a handle 23!) to allow an operatar to rotate it as desired. From' the discussion which has preceded it will be seen that unless t'1e key plate is at the proper angular position, at the most only occasional pulses at irregular intervals will pass the band pass filter referred to, and the operator will accordingly hear no intelligible signal in his head phones. Therefore, when, as a result of moving the key plate 23 by means of the handle 23b he does hear an intslligible signal, he will know that the receiver is properly tuned, since unless there is more than one emitter with the same key plate, it would be impossible for him to receive intelligibly a signal sent out by any other emitter. It will be appreciated also, that when the key plate 23 is -in the proper angular position the desired signal If the condition mentioned at the beginning of will be received but any jamming signal which Although enemy jamming signals will on the whole be rejected, yet unless steps are taken to prevent this, they may cause some disturbance in the desired received signals. This can best be illustrated by an example. Assume that only onewave of constant frequency is used at the emitter and that its frequency is 10,000 cycles. Assume also that at one instant the rate of scanning by the electron beam 31a was such that the frequency of a wave passing through the wire 50 and resulting from a jamming signal, was 9,000

cycles, but that as a result of the increase in the scanning speed this frequency increased in a given short period of time to 11,000 cycles. In passing from the lower to the higher frequency the jamming wave would at one moment have had the same frequency as the desired constant frequency wave and would thus pass the band pass filter. If its amplitude at this moment were suficient, a sound which might be confused with a Morse dot might thus be produced in the head phones. Any really serious efiect of this could be avoided by using instead of the ordinary Morse system, where the signals are composed of dots and dashes, a modified system where the signals are composed of long and short dashes so that dots would have no significance in the received message (it is for this reason that reference has been made to a Morse type message), but such a departure from normal signalling practice might be dificult.

Figure 4 shows an arrangement at the receiver by which all or practically all of the extraneous dots mentioned may be eliminated without the elimination of any Morse dots, and it is for the purpose of this arrangement that the use in the emitter of two constant frequency waves instead of only one has been illustrated and described. The arrangement is based upon the fact that extraneous dots will never appear at the same instant in the two constant frequency waves unless a number of jamming signals have been sent out by an enemy and one of these, after distortion by the variable erasing speed, happens to be momentarily at the frequency of one of the desired constant frequency waves at the instant that another is at the frequency of the other of such waves.

According to the arrangement in question, two band pass filters 51 and 58 are provided, one designed to pass a wave of the frequency of that produced by the oscillator in and the other to pass a wave of the frequency of that produced by the oscillator I22). The final receiving device for the desired signals, shown for the sake of example as a pair of headphones 59, is connected in the plate circuit of a four electrode tube 60 having two grids 6i and 62, of which the grid BI is fed with current passing through the filter 51 and the grid 62 is fed by rectified current which has passed through the other filter 58. This latter grid is connected to a negative biassing battery 64 through a load resistor 53, which is in a circuit 65 including a rectifier 68 so disposed as to pass current through the resistor only in a direction opposing the polarity of the battery 64. When current, representing for instance a Morse dot or dash, passes the filter 58, the resulting rectified current flow in the circuit 66 will produce a potential on the grid 62 opposing that of the battery 64 and will thus unbias the grid to allow passage of electrons from the cathode 61 to the plate 68. A condenser 63a may be shunted around the resister 63 in order to keep the grid potential as uniform as possible during passage of current through the circuit 65.

From the above description it will be seen that a signal will be heard in the headphones 59 only when current flows simultaneously through both the filters 51 and 58, that is practically only when a Morse dot or dash is received. If current flows only through the filter 58, then there will be no modulation of the current in the plate circuit of the tube 60, because the potential of the grid 6| will stay constant. If, on the other hand, it flows only through the filter 51, then there will be no current at'allin the plate circuit because the grid 62 will be at its normal biassed potential. dot will be cut out.

It may be pointed out that the invention is applicable to multiplex communication and could, for example, be used in systems of the type shown in application Serial No. 252,030, filed January 20, 1939, or application Serial No. 381,906, filed March 5, 1941.

What I claim is:

1. The method of hindering interference with wireless transmission of messages, which com prises distorting a wave adapted for selection from all other waves by means of a filter to a wave unadapted for 'such selection, modulating a carrier wave with such distorted wave itself modulated in accordance with a message, and

simultaneously modulating said carrier Wave witha different wave unadapted for selection from all other waves by means of a filter.

2. The method of hindering interference with wireless transmission of Morse type messages, which comprises distorting wave adapted for selection from all other waves by means of a filter to a wave unadapted for such selection; modulating a carrier wave with such distorted wave itself modulated in accordance with a Morse type message, and simultaneously modulating said carrier wave with a different wave unadapted for selection from all other waves by means of a filter.

3..The method of hindering interference with wireless transmission of Morse type messages, which comprises distorting a wave adapted for selection from all other waves by means of a filter to a wave unadapted for such selection, modulating a carrier wave with such distorted wave itself modulated in accordance with a Morse type message, and simultaneously modulating said carrier wave with a different wave unadapted for selection from all other waves by means of a filter and having variations of amplitude of the nature of those obtained by modulation with a Morse type message.

4. The method of hindering interference with wireless transmission of messages, which comprises distorting a plurality of diiferent waves each adapted for selection from all other waves by means of a filter to waves unadapted for such selection, modulating a carrier wave with such distorted Waves themselves modulated in accordance with a message, and simultaneously modulating said carrier wave with a different wave means of a filter.

Thus, in-either case, the extraneous 5. The method of hindering interference with wireless transmission of messages, which comprises distorting a plurality of different waves each adapted for selection from all other waves by means of a filter to waves unadapted for such selection, modulating a carrier wave with such distorted waves themselves modulated in accordance with a message, and simultaneously modulating said carrier wave with a different wave unadapted for selection from all other waves by means of a filter and having variations of amplitude of the nature of those obtained by modulation with a Morse type message.

6. The method of hindering interference with wireless transmission of messages, which comprises distorting a wave of constant frequency to one of variable frequency, modulating a carrier wave with such distorted wave itself modulated in accordance with a message, and simultaneously modulating said carrier wave with another wave of differently variable frequency.

7'. The method of hindering interference with wireless transmission of Morse type messages,

which comprises distorting a wave of constant frequency to one of variable frequency, modulating a carrier wave with such distorted wave itself modulated in accordance with a Morse type message, and simultaneously modulating said car rier wave with another wave of differently variable frequency.

8. The method of hindering interference with wireless transmission of Morse type messages, which comprises distorting a wave of constant frequency to one of variable frequency, modulat wave itself 10. The method of hindering interference with wireless transmission of Morse type messages, which comprises distorting a wave of constant frequency to one of variable frequency, modulating a carrier wave with such distorted wave itself modulated in accordance with a Morse type message, and simultaneously modulating said carrier wave with a substantial aperiodic'wave having variations of amplitude of the nature of those obtained by modulation with a Morse type message. v

11. The method of hindering interference with wireless transmission of messages, which comprises recording a wave of constant frequency, converting the resulting record at a variable speed into an electric wave of variable frequency, modulating a carrier wave with said electric wave itself modulated inaccordance with a message, and simultaneously modulating such carrier wave with another wave of differently variable frequency.

12. The method of hindering interference with wireless transmission of Morse type messages, which comprises recording a wave of constant frequency, converting the resulting record at a variable speed into an electric wave of variable frequency, modulating a carrier wave with said electric wave itself modulated in accordance with a Morse type message, and simultaneously modulating such carrier wave with another wave of differently variable frequency.

13. The method of hindering interference with wireless transmission of Morse type messages, which comprises simultaneously recording a plurality of waves ofconstant frequency, converting the resulting record at a variable speed into a composite electric wave of variable frequency, modulating a carrier wave with such electric wave itself modulated in accordance with a Morse type message, and simultaneously modulating such carrier wave with another wave of differently variable frequency.

14. Apparatus for hindering interference with wireless transmission of messages, which comprises means for distorting a fllterable wave to an unfilterable wave, means for producing another unfilterable wave, and means for modulating a carrier wave simultaneously with said other unfilterable wave and with said first mentioned unfilterable wave itself modulated in accordance with a message.

15. Apparatus for hindering interference with wireless transmission of Morse type messages, which comprises means for distorting a filterable wave to an unfilterable wave, means for producing another unfilterable wave, and means for modulating a carrier wave simultaneously with said other unfilterable wave and with said first mentioned unfilterable wave itself modulated in accordance with a Morse type message.

16. Apparatus for hindering interference with wireless transmission of m ssages, whic ccm= prises a mosaic, means for producing a beam of electrons, means for modulating the intensity of said beam with an electric wave of a constant frequency, means for scanning said mosaic with said beam to form an electrostatic record of the variations in intensity of the beam, means for scanning said mosaic at a variable speed to convert said record into an electric wave of variable frequency, means for causing said wave of variable frequency to be modulated in accordance with a message, means for producing another electric wave of differently variable frequency, and means for modulating a carrier wave simultaneously with both said waves of variable frequency.

17. Apparatus for hindering interference with wireless transmission of Morse type messages which comprises a mosaic,,means for producin a beam of electrons, means for modulating the intensity of said beam with an electric wave of a constant frequency, means for scanning said mosaic with said beam toform an electrostatic record of the variations in intensity of the beam, means for scanning said mosaic at a variable speed to convert said record into an electric wave of variable frequency, means for causing said wave of variable frequency to be modulated in accordance with a Morse type message, means for producing another electric wave of differently variable frequency, and means for modulating a carrier wave simultaneously with both said waves .of variable frequency.

18. In apparatus of the class described, means 1 a cathode and an anode connected to said second ing an electron beam which forms a luminous spot where it strikes said fluorescent coating, a screen of variably increasing transiucency from a predetermined minimum to a predetermined maximum interposed between said fluorescent coating and said cathode, means for focussing on said cathode light passing through said screen of variable translucency, and means for causing said electron beam to scan said fluorescent screen from the point opposite the point of minimum translucency on said screen of variable translucency to the point opposite the point of maximum translucency on said last mentioned screen.

19. Apparatus for receiving signals composed of two waves of constant frequency distorted to waves of variable frequency with their amplitudes varied in accordance with a Morse type message, comprising means for reversing the distortion of the waves to reconstitute the waves of constant frequency, filters for said waves of constant frequency, a multi-electrode tube having a filament, a plate. a first grid and a second grid, means for feeding one of said reconstituted waves of constant frequency to said first grid, means for biasing said second grid normally to block passage of electrons from said filament to said plate, and means for rectifying the other waveof constant frequency and feeding the rectifled current to said second grid to neutralize. the

a bias thereof. 

